Embedded capacitors are being developed by the semiconductor industry to address increasing demands for miniaturization and faster clock speeds. Such devices consist of a high permittivity dielectric layer sandwiched between a foil substrate electrode and a top electrode, where both electrodes are typically copper. The coating of a thin dielectric layer onto a metallic foil can be accomplished by processes such as reactive sputtering, laser ablation, metallo-organic chemical vapor deposition, liquid source misted chemical deposition, chemical solution deposition, spin coating or spraying, or dipping. In substantially all cases, however, the deposited dielectric layer is thermochemically processed after deposition by heat treating the organometallic precursors therein to first remove their organic molecular constituents, and subsequently to sinter and crystallize the inorganic residue to provide a dielectric layer with useful properties.
In manufacturing a thin film capacitor (“TFC”), where dielectric oxide precursors are deposited on a metal foil electrode, it is desirable to be able to process the dielectric oxide precursor formulation in oxygen and at a high temperature because the dielectric layer needs oxygen during its thermal processing so that a dielectric layer with dense, large grains is obtained that will be characterized by high permittivity, low loss and high insulation resistance. At elevated oxygen pressures, however, there is a risk that the metal in the substrate will oxidize, and high temperatures may approach the melting point of the metal in the substrate and/or cause the metal to lose its integrity.
Moreover, the high temperature processing of the dielectric layer may induce the formation of defects within the dielectric, and/or at the dielectric/substrate interface, resulting from microstructural and chemical changes experienced by the metal foil substrate. Grain boundaries intersecting the surface of a metal foil substrate on which a dielectric layer is deposited can act as defect sources during the synthesis of the dielectric layer.
Copper-based foil substrate electrodes are preferred because of their ready availability, higher electronic conductivity, and the vast know-how in the industry relative to integration of this material in the form of an embedded component. However, the poor oxidation resistance and lower melting point of copper-based compositions, and their tendency toward thermal migration and outgassing, seriously reduce the upper limits in oxygen partial pressure and processing temperature that can be used to manufacture a TFC on a copper substrate. For example, copper electrical conductors experience oxidation when held in air at temperatures higher than 100° C., which results in decreases in conductivity and strength. In addition, when copper is used as a thin metallic foil, the thin cross section weakens the mechanical robustness of the foil making it quite susceptible to handling damage in the form of wrinkles as a result of fabrication steps prior to, during and after the deposition of the dielectric layer.
To address some of the deficiencies of copper-based foil substrates, substrates have been prepared with other metals such as nickel. For example, U.S. Pat. No. 6,841,080 discloses a method of forming a multi-layer foil comprising depositing a barrier layer of nickel phosphorus on a conductive metal layer by a metal deposition method selected from the group consisting of electroless plating, electrolytic plating, sputtering or vacuum plating.
U.S. Pat. No. 6,541,137 discloses a multi-layer foil comprising a copper layer of about 10 to about 50 μm thickness having a barrier layer composing about 1 to about 3 μm thickness nickel phosphorus disposed on one of the copper foil faces, wherein the phosphorus concentration of the nickel phosphorus layer is about 4 to about 11 wt %.
U.S. Pat. No. 6,649,930 discloses a nickel-coated copper foil substrate. And, U.S. Pat. No. 7,190,016 discloses a barrier layer having a thickness of 0.5 to 2 μm disposed on each of the faces of a copper foil, wherein the barrier layer is an electrodeposited nickel layer comprising less than 3 atomic percent of copper.
A need nevertheless remains for materials and techniques that can be used to provide a high permittivity TFC without hindrance from the problems caused by copper as a choice of substrate metal.